Experimental studies indicate that intermittent synchronization across large cortical areas provides the window for the emergence of meaningful cognitive activity in animals and humans [1, 2]. In neural tissues, populations of neurons send electric currents to each other and produce activation potentials observed in electroencephalography (EEG) experiments. The synchrony among neural units can be evaluated by comparing their activation levels as the function of time. While single unit activations have large variability and do not seem synchronous, the activations of neural groups often exhibit apparent synchrony.
Experimental studies on brain waves at the level of neural populations using EEG techniques gave rise to new theories. Multiple electrode recordings in the olfactory bulb indicated that odors are encoded as complex spatial and temporal patterns in the bulb. Based on these observations, a chaos theory of sensory perception has been proposed [3, 4]. In this approach, the state variables of the brain in general, and the olfactory bulb in particular, traverse along complex chaotic trajectories which constitute a strange attractor with multiple wings. External stimuli constrain the trajectories to one of the attractor wings, which are identified as stimulus specific patterns. Once the stimulus disappears, the dynamics returns to the unconstrained state until the next stimulus arrives.
EEG measurements confirm the presence of the self-sustained, randomized, steady state background activity of brains. This background activity is the source from which ordered states of macroscopic neural activity emerge, like the patterns of waves at the surfaces of deep bodies of water. Neural tissues, however, are not passive media, through which effects propagate like waves in water [5]. The brain medium has an intimate relationship with the dynamics through a generally weak, subthreshold interaction of neurons. The brain activity exhibits high level of synchrony across large cortical regions. The synchrony is interrupted by episodes of desynchronization, when propagation of phase gradients in the activation of local populations can be identified. The spatially ordered phase relationship between cortical signals is called phase cone. Experiments show that phase gradients propagate at velocities up to 2 m/s and can cover cortical areas of several cm2 [1, 6].